The configuration of a typical Bluetooth receiver is introduced in FIG. 1. Referring to FIG. 1, the Bluetooth receiver includes a front-end circuit 110 configured to obtain a baseband signal by demodulating a received signal, a filter 120 configured to selectively pass a desired channel therethrough and remove an undesired channel, and a recovery circuit 130 configured to convert the received baseband signal in the frequency domain into time domain symbols.
A Bluetooth transmitter generates a carrier frequency-based modulated signal by modulating a baseband signal. A Bluetooth Smart transmitter uses a Gaussian frequency shift keying modulation method that has a modulation index h ranging from 0.45 to 0.55. The frequency shift keying method refers to a method of transmitting digital information through the variation of the discrete frequency of a carrier.
FIG. 3 is a diagram showing the frequency characteristic of a modulated signal that is transmitted by a Bluetooth transmitter in an ideal environment in which noise and frequency offset are not present. Referring to FIG. 3, there are shown the minimum and maximum frequency shifts of a signal having a symbol transmission speed Fs of 1 Msps and a modulation index h of 0.5 that is transmitted via a carrier frequency Fc in the 2.4 GHz band. When the symbol transmission speed Fs is 1 Msps, the signal of the bit value “1” corresponding to the symbol “+1” has a frequency shift F+ of +250 kHz (a frequency shift in a positive (+) direction) from a center frequency Fc, the signal of the bit value “0” corresponding to the symbol “−1” has a frequency shift F− of −250 kHz (a frequency shift in a negative (−) direction) from the center frequency Fc.
Referring back to FIG. 1, the front-end circuit 110 of the Bluetooth Smart receiver obtains a frequency-demodulated waveform in a baseband by using an analog or digital frequency demodulator, and estimates transmission bit information by deciding signs at symbol intervals.
FIG. 4 is an eye diagram in which frequency-demodulated waveforms are accumulated at symbol intervals. A Bluetooth Smart receiver obtains a frequency-demodulated waveform in a baseband by using an analog or digital frequency demodulator. When frequency-demodulated waveforms are accumulated at symbol intervals, an eye diagram, such as that of FIG. 4, is obtained. A point at which the gap between a frequency shift in a positive (+) direction and a frequency shift in a negative (−) direction is greatest corresponds to an optimum symbol timing phase at which inter-symbol interference is lowest. Errorless transmission bit information is obtained by performing sign decision at symbol intervals while continuously maintaining an optimum timing phase.
Since a signal is received in the state in which the quality thereof has been degraded due to signal magnitude offset, carrier offset, timing offset, etc. attributable to mismatch between a transmitter and the receiver, the receiver must be prepared for errorless bit demodulation by implementing a recoverer for corresponding offset.
FIG. 2 is a diagram showing a typical packet of Bluetooth Smart. Referring to FIG. 2, the packet of Bluetooth Smart includes a preamble interval 210, an access address interval 220, a protocol data unit (PDU) interval 230, and a CRC interval 240. Since a Bluetooth receiver must identify an address during the access address interval 220 and must identify and process data during the PDU interval 230, preparation for the identification of the address and the data must be completed during the preamble interval 210. Accordingly, there is a time limitation in that operations, such as automatic gain control, frequency offset compensation, timing compensation, etc., must be performed within a preamble interval of Bluetooth or Bluetooth Smart in the front-end circuit 110 of the Bluetooth receiver.
For a receiver to estimate offset, a previously agreed upon pilot signal is required between a transmitter and the receiver. According to the Bluetooth Smart standard, a bit stream corresponding to the start of a packet is transmitted in the preamble interval 210. The bit stream of the preamble interval 210 is determined by the first transmission bit of the access address interval 220. When the first transmission bit of the access address interval 220 is “1,” the bit stream value “01010101b” of the preamble interval 210 is transmitted. When the first transmission bit of the access address interval 220 “0,” the bit stream value “10101010b” of the preamble interval 210 is transmitted. Since the frequency-demodulated waveform of the preamble interval 210 has a sine wave-like form in which negative (−) and positive (+) frequency shifts are repeated, it has a characteristic considerably appropriate for estimating symbol timing offset, and is appropriate for being used as a pilot signal. Initial symbol timing acquisition is performed to obtain the correlation between a stored preamble and a received signal and to estimate a point at which a peak occurs as an optimum symbol timing.
An example of a preceding technology for compensating for the symbol timing offset of a received signal in a Bluetooth receiver is disclosed in U.S. Pat. No. 8,401,120 entitled “Symbol Error Acquisition for Bluetooth Enhanced Data Rate Packets.”
The preceding technology is configured to detect a phase error by first acquiring an initial timing during a preamble interval and then monitoring changes in timing in the following protocol data unit (PDU) interval. That is, the preceding technology is a technology configured to detect a phase error by comparing the phase of the output symbol of a symbol demodulator with the phase of a received signal, to provide notification that a current symbol timing is not reliable when the phase error exceeds a threshold value, and to compensate for a symbol timing error.
However, according to the preceding technologies, inter-symbol interference occurs due to a Gaussian filter during the frequency shift keying process of the Bluetooth Smart standard and thus a frequency shift waveform is distorted, and the jitter of a timing error detector occurring due to the randomness of a data symbol in access address and protocol data unit intervals causes a significant problem. In particular, in a current situation in which a demand for a high-sensitivity receiver supporting a value equal to or lower than −90 dBm is increasing, it is difficult to perform sufficient offset compensation on a Bluetooth Smart signal by using the conventional preceding technologies. Therefore, there is an increasing need for a means that is capable of dealing with this situation.